On cellular radio networks data and speech are transmitted in the circuit-switched form, but nowadays packet-switched transmission is also possible. GPRS (General Packet Radio Service) is a GSM-based (Global System for Mobile Communications) service in which air interface capacity not in use in circuit-switching is used for packet transmission. The basic GPRS uses GMSK (Gaussian Minimum-Shift Keying) as the modulation method.
EGPRS (Enhanced GPRS) uses EDGE (Enhanced Data Rates for GSM Evolution) technology to increase data transmission capacity. In addition to the GMSK modulation normally in use in the GSM, 8-PSK (8-Phase Shift Keying) modulation can be applied to packet data channels. The main object is to provide non-real-time data transmission services, such as file copying and Internet browser use, but also real-time services for packet-switched transmission of speech and video images. In principle, the data transmission capacity may vary from a few kilobits per second even up to 400 kbit/s.
The quality meters of packet-switched services are advanced, whereas the quality meters used for circuit-switched services are less advanced. The quality of circuit-switched services is not actually measured; rather, the raw bit error ratio of the traffic channel, i.e. radio link, that implements the service is measured.
Examples of such quality values are parameters RX_QUAL_FULL and RX_QUAL_SUB used in the GSM system. The RX_QUAL_FULL gives the bit error ratio estimate of hundred frames included in four 26-multiframes, and the RX_QUAL_SUB gives the bit error ratio estimate of associated control channel frames and silence descriptor frames of four 26-multiframes transmitted during discontinuous transmission, i.e. the bit error ratio estimate of twelve frames altogether. The problem associated with these parameters is that they do not indicate the actual quality of service because the final bit error ratio is dependent on the channel coding used for the service, which is not taken into account in the calculation of these parameters. Another problem is that the transmitter does not necessarily know whether non-continuous transmission is used, and consequently these parameters may be given values that do not correspond to the real situation.
In the EGPRS a bit error probability is calculated for a traffic channel in packet transmission on the basis of soft bit decisions of a Viterbi decoder, for example. This can be implemented e.g. so that the first parameter BEP_MEAN indicates the average of bit error probability in an RLC (Radio Link Control) block and the second parameter BEP_STD indicates the standard deviation of bit error probability in the RLC block. When applied to circuit-switched transmission, the problem related to these parameters is that they are only estimates which reflect the raw bit error ratio.